1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter apparatus, and more particularly, to a SAW filter apparatus having a balanced-to-unbalanced conversion function and having different impedance characteristics at an input side and an output side of the SAW apparatus. The present invention also relates to a communication apparatus including the above-described SAW filter apparatus.
2. Description of the Related Art
There has been significant technological progress in decreasing the size and the weight of portable communication apparatuses, such as cellular telephones. One way to achieve such results is to reduce the number and the size of the individual components of the communication apparatus. Additionally, components having composite functions are being developed.
In view of this background, research is being actively conducted with respect to a SAW filter apparatus used in an RF stage provided with a balanced-to-unbalanced conversion function (so-called “balun function”). Such SAW filter apparatuses are being mainly used in a global system for mobile communications (GSM).
Generally, in a communication apparatus, an unbalanced signal having a characteristic impedance of 50 Ω is used for a portion from an antenna to a filter. For an amplifier, which is the subsequent stage of the filter, a balanced signal having an impedance of 150 Ω to 200 Ω is usually used.
As a SAW filter apparatus provided with a balun function for converting a 50 Ω unbalanced signal into a 150 Ω to 200 Ω balanced signal, a SAW filter apparatus which implements an unbalanced input and a balanced output by using four SAW filter devices is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 10-117123. The configuration of the SAW filter apparatus disclosed in this publication is shown in FIG. 8.
The SAW filter apparatus shown in FIG. 8 includes two SAW filter units 511 and 512. The SAW filter unit 511 is constructed by cascade-connecting SAW filter devices 501 and 502 having equal phase characteristics, while the SAW filter unit 512 is constructed by cascade-connecting SAW filter devices 503 and 504, having transmission phase characteristics that are about 180° out of phase with each other. The input terminals of the SAW filter units 511 and 512 are connected in parallel to each other by an unbalanced terminal 505, while the output terminals of the SAW filter units 511 and 512 are connected in series to each other by balanced terminals 506 and 507.
In the SAW filter apparatus provided with the above-described balanced-to-unbalanced input/output function, the outputs from the balanced terminals 506 and 507 are used as a difference between the balanced terminals 506 and 507. Accordingly, the maximum output can be obtained when the electrical signals from the balanced terminals 506 and 507 are 180° out of phase with each other. Conversely, when the electrical signals from the balanced terminals 506 and 507 are in phase with each other, they cancel each other out. Accordingly, a higher level of attenuation can be obtained as the two electrical signals have closer levels.
Thus, the SAW filter apparatus is desirably configured so that the outputs of the balanced terminals 506 and 507 are 180° out of phase with each other in the pass band, and they are in phase with each other in the stop band (other than the pass band).
In the SAW filter apparatus disclosed in Japanese Unexamined Patent Application Publication No. 10-117123, four SAW filter devices are used, and for inverting the phase of one of the SAW filter devices, the directions of the comb-like electrodes (interdigital transducers: hereinafter simply referred to as “IDTs”) are inverted with respect to the SAW propagating direction as the symmetric axis, or the pitch between the IDTs of one of the SAW filter devices 511 and 512 is increased by 0.5 λ (wavelength).
With this configuration, the phase characteristics of the balanced terminals 506 and 507 are inverted in the pass band, while they are in phase with each other in a frequency band in which a SAW is not excited.
In the above-configured SAW filter apparatus, however, spurious responses are generated in a region in the vicinity of the pass band due to the excitation of a SAW, and the phase characteristics of the balanced terminals 506 and 507 in the region in which spurious responses are generated are disadvantageously inverted with respect to each other, as in the pass band. Thus, signals output from the balanced terminals 506 and 507 do not cancel each other out, and the attenuation in the frequency region other than the pass band is not sufficient.